PSA processes are commonly used for the production of oxygen when small to intermediate quantities, i. e., up to 100 TPD (tons per day) of oxygen is required. Typical small to intermediate size plants use two-bed or three-bed adsorption systems.
However, when quantities greater than 100 TPD are to be produced by PSA, it is generally necessary to use systems comprised of four or more beds, because very large two-bed or three-bed systems are too costly to construct, ship and operate. Typically, power consumption of small bed units is currently between about 0.35 and 0.45 Kwh/NmO.sub.2 (kilowatt hours per normal cubic meter of oxygen produced). In order for large volume PSA systems to compete with cryogenic air separation units of the same capacity, they must have low power consumption demand. To be competitive, it is generally considered that large scale PSA plant power demand should not significantly exceed about 0.45 Kwh/NmO.sub.2.
Efforts to reduce the operating cost of large scale PSA plants are constantly made. U.S. Pat. No. 5,656,068 discloses a four bed PSA plant in which the beds are operated as two pairs of 2-bed systems, which is referred to as a 2.times.2 cycle/system. Each pair of beds of the disclosed system is operated 180.degree. out of phase and the two pairs of beds are operated out of phase by one-half of a half-cycle. A stated advantage of the process of this patent is that it uses a combination of positive displacement and centrifugal pumps to perform the various gas pumping operations. The system requires two air compressors, one of which periodically is in the idle or vent mode, and two vacuum pumps.
The operating costs of a four-bed PSA plant could be considerably reduced if the plant could be operated in such a manner that fewer than four pumps were required. The present invention provides a system and operating cycle that possesses this advantage.